Martin Schoell

Genetic Characterization of Natural Gases

Natural gases can be characterized genetically using four properties: C2+ concentration, carbon and hydrogen isotope variations in methane, and carbon isotope variation in ethane. Three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters. In these diagrams, compositional fields have been defined for primary gases (biogenic, thermogenic associated, and thermogenic nonassociated) and for gases which result from mixing of these gases. These fields are strictly empirical and comprise compositional variations found in about 500 natural gases. The isotopic and compositional variations in natural gases can be described in terms of (1) processes during formation of the gases such as bacterial fermentation or maturation of organic matter, and (2) processes during secondary migration. Mixing of primary gases is an important and common process. Migration of gases predominantly affects the C2+ concentration, whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged, allowing an assessment of the origin of migrated gases and properties of their source rocks. The formation of gas from humic organic matter and coals is not yet clear from published data. The diagrams use data from various basins and areas. Interstitial gases from the Gulf of California are entirely of bacterial origin: traces of thermogenic gases are formed only in the vicinity of dolerite sills; gases in the south German Molasse basin and in the Vienna basin are of bacterial, mixed, and thermogenic origins. Data from the north Italian Po basin provide examples for genetic characterization of migrated gases.

Multiple origins of methane in the Earth

Methane occurrences in the Earths crust are predominantly of biogenic origin, i.e. their ultimate source is biologically formed organic matter. Methane can also form through inorganic reactions and is consequently termed abiogenic. Biogenic methanes can either form through bacterial or thermogenic processes. Bacterial processes follow a CO2 reduction and/or fermentation pathway. The fermentation processes are quantitatively more important in recent fresh sediments and swamps. Methane formed by CO2 reduction, however, is most common in older sediments and commercial gasfields. Temperature, organic substrate and age may be the major factors controlling the relative importance of the two pathways. Stable-isotope concentrations in thermogenic methanes seem to be controlled by the extent of conversion of organic matter, the timing of gas expulsion, and trapping. The different character of methane in individual sedimentary basins may be a result of the geologic history. Geothermal methanes are most likely derived from pyrolysis of organic matter. Abiogenic methane occurs in hydrothermal vents and ophiolite complexes. Inorganic reactions, either surficial or deep-seated, are the likely source of such methanes. A uniform mantle origin of methane is not supported by the observed isotope variations in naturally occurring methanes.

Sensitivity of biomarker properties to depositional environment and/or source input in the Lower Toarcian of SW-Germany

Abstract A sequence of Lower Toarcian sediments which are highly contrasting in their depositional environment over a 5 m depth interval has been investigated in detail for the variability of geochemical properties which are used for source characterization and oil-to-source correlation. Lower Liassic marls are representative of a well-aerated shallow-water environment in contrast to the immediately overlying bituminous shales which are deposited under extremely reducing conditions. Various properties have been found to vary considerably within these two units. Amongst the most important are carbon isotopic composition of the kerogen, the pristane/phytane ratios, nickel as opposed to vanadyl porphyrins, and the C27 dia-/regular steranes. Although maturation within the profile does not change, some of the maturation-dependent biomarker properties such as the monoaromatic steroid side-chain cracking and the Tm/Ts ratio exhibit large changes which can be assigned to diagenetic processes. Another maturation-dependent property, the 20S/20R epimerization of C29 steranes, exhibits smaller changes which could also be due to early diagenetic processes. The study suggests that reducing and oxidizing conditions, i.e. Eh and pH in the sediment, exert an influence on several biomarker precursor-product pathways. In maturation studies initial variations due to depositional conditions have therefore to be taken into account.

Solubility trapping in formation water as dominant CO2 sink in natural gas fields

Injecting CO2 into deep geological strata is proposed as a safe and economically favourable means of storing CO2 captured from industrial point sources. It is difficult, however, to assess the long-term consequences of CO2 flooding in the subsurface from decadal observations of existing disposal sites. Both the site design and long-term safety modelling critically depend on how and where CO2 will be stored in the site over its lifetime. Within a geological storage site, the injected CO2 can dissolve in solution or precipitate as carbonate minerals. Here we identify and quantify the principal mechanism of CO2 fluid phase removal in nine natural gas fields in North America, China and Europe, using noble gas and carbon isotope tracers. The natural gas fields investigated in our study are dominated by a CO2 phase and provide a natural analogue for assessing the geological storage of anthropogenic CO2 over millennial timescales. We find that in seven gas fields with siliciclastic or carbonate-dominated reservoir lithologies, dissolution in formation water at a pH of 5–5.8 is the sole major sink for CO2. In two fields with siliciclastic reservoir lithologies, some CO2 loss through precipitation as carbonate minerals cannot be ruled out, but can account for a maximum of 18 per cent of the loss of emplaced CO2. In view of our findings that geological mineral fixation is a minor CO2 trapping mechanism in natural gas fields, we suggest that long-term anthropogenic CO2 storage models in similar geological systems should focus on the potential mobility of CO2 dissolved in water.

Mathematical modeling of stable carbon isotope ratios in natural gases

Abstract A new approach is presented for mathematical modeling of stable carbon isotope ratios in hydrocarbon gases based on both theoretical and experimental data. The kinetic model uses a set of parallel first-order gas generation reactions in which the relative cracking rates of isotopically substituted (k∗) and unsubstituted (k) bonds are represented by the equation k∗/k=(A f ∗/A f ) exp(−ΔEa/RT), where R is the gas constant and T is temperature. Quantum chemistry calculations have been used to estimate the entropic (Af∗/Af) and enthalpic (ΔEa) terms for homolytic bond cleavage in a variety of simple molecules. For loss of a methyl group from a short-chain n-alkane (≤ C6), for example, we obtain an average ΔEa of 42.0 cal/mol and an average Af∗/Af of 1.021. Expressed differently, 13C-methane generation is predicted to be 2.4% (24‰) slower than 12C-methane generation (from a short-chain n-alkane) in a sedimentary basin at 200°C but only 0.7% (7‰) slower in a laboratory heating experiment at 500°C. Similar calculations carried out for homolytic bond cleavage in other molecules show that with few exceptions, ΔEa varies between 0 and 60 cal/mol and Af∗/Af between 1.00 and 1.04. Examination of this larger data set reveals: (1) a weak sigmoid relationship between ΔEa and bond dissociation energy; and (2) a strong positive correlation between ΔEa and Af∗/Af. The significance of these findings is illustrated by fitting a kinetic model to chemical and isotopic data for the generation of methane from n-octadecane under isothermal closed-system conditions. For a specific temperature history, the fitted model provides quantitative relationships among methane carbon isotope composition, total methane yield and methane generation rate which may have relevance to the cracking of oil-prone kerogens and crude oil. The observed variability of the kinetic reactivity of various methane source rocks highlights the need to apply and adequately calibrate such models with laboratory data for specific study areas. With this approach isotope data of natural gases can be used not only to estimate the time of gas generation in a sedimentary basin, but also to evaluate the source rock maturities at which specific accumulations were generated, and place constraints on trap charging histories.

Genetic and temporal relations between formation waters and biogenic methane: Upper Devonian Antrim Shale, Michigan Basin, USA

Abstract Controversy remains regarding how well geochemical criteria can distinguish microbial from thermogenic methane. Natural gas in most conventional deposits has migrated from a source rock to a reservoir, rarely remaining associated with the original or cogenetic formation waters. We investigated an unusual gas reservoir, the Late Devonian Antrim Shale, in which large volumes of variably saline water are coproduced with gas. The Antrim Shale is organic-rich, of relatively low thermal maturity, extensively fractured, and is both source and reservoir for methane that is generated dominantly by microbial activity. This hydrogeologic setting permits integration of chemical and isotopic compositions of coproduced water and gas, providing a unique opportunity to characterize methane generating mechanisms. The well-developed fracture network provides a conduit for gas and water mass transport within the Antrim Shale and allows invasion of meteoric water from overlying aquifers in the glacial drift. Steep regional concentration gradients in chemical and isotopic data are observed for formation waters and gases; dilute waters grade into dense brines (300,000 ppm) over lateral distances of less than 30 km. Radiogenic (14C and 3H) and stable isotope (18O and D) analyses of shallow Antrim Shale formation waters and glacial drift groundwaters indicate recharge times from modern to 20,000 yr bp . Carbon isotope compositions of methane from Antrim Shale wells are typical of the established range for thermogenic or mixed gas (δ13C = −47 to −56‰). However, the unusually high δ13C values of CO2 coproduced with methane (∼+22‰) and dissolved inorganic carbon (DIC) in formation waters (∼+28‰) require bacterial mediation. The δD values of methane and coproduced formation water provide the strongest evidence of bacterial methanogenesis. Methane/[ethane + propane] ratios and δ13C values for ethane indicate: (1) the presence of a thermogenic gas component that increases basinward and (2) progressive bacterial oxidation of ethane as the Antrim Shale subcrop is approached. Multiple episodes of Pleistocene glaciation over northern Michigan appear critical to the development of these gas deposits. Loading of thick ice sheets may have provided hydraulic head that enhanced dilation of preexisting fractures and influx of meteoric water. The physical erosion cycle of repeated glacial advances and retreats exhumed the Antrim Shale around the northern margin of the Michigan Basin, subjecting it to near-surface physiochemical and biochemical processes. The chemical and hydrologic relations demonstrated in the Antrim Shale reservoir suggest a dynamic connection between Pleistocene glacial history of the midcontinent region and development of recoverable, microbially generated natural gas reserves.

Rearranged hopanes in sediments and petroleum

Abstract Two new rearranged hopanoid hydrocarbons have been isolated from a Prudhoe Bay crude, Alaska. 17α(H)-15α-methyl-27-norhopane was determined by X-ray crystallography. It is the first identified member of a new series of rearranged hopanes we propose to call “17α(H)-diahopanes.” Analysis by gas chromatography-mass spectrometry—mass spectrometry (GC-MS-MS) of the parents of m/z 191 in several crudes suggests that this compound is a member of a C 29 -C 34 series of 17α(H)-diahopanes common to many crude oils and sediments. In addition, a new member of the 18α(H)-neohopane series has also been elucidated. Determination of 18α(H)-17α-methyl-28,30-dinorhopane [18α(H)-30-norneohopane ], which we propose to nickname “C 29 Ts,” hinged upon advanced nuclear magnetic resonance (NMR) techniques (at 500 and 600 MHz) such as proton-detected 1 H- 13 C correlated spectra for the C-skeleton and Rotating-frame Overhauser Enhancement Spectroscopy (ROESY) for stereochemistry, as well as several other two-dimensional (2D) NMR techniques. This compound is the second known pseudohomolog of the neohopane series (together with 18α(H),22,29,30-trisnorneohopane, Ts), but the existence of additional pseudohomologs is still not clear. The structures of these rearranged hopanes are consistent with an origin by catalytic rearrangement from hopenes during early diagenesis. Carbon isotopic data collected on Ts, 17α(H)-diahopane, C 29 Ts, 17α(H)-22,29,30-trisnorhopane (Tm), 17α(H)-30-norhopane, and 17α(H)-hopane isolated from the Prudhoe Bay oil are in the -27 to -28%o δ 13 C range supporting mechanistic arguments based on structures that all are derived from common precursors. These δ 13 C values are slightly more positive than the whole Prudhoe Bay oil (-30.1%), suggesting that these hopanes may have been derived from heterotrophic or cyanobacteria in the paleoecosystem during deposition of its source rock. Molecular mechanics calculations predict relative thermal stabilities in the order 17α(H)-diahopanes > 18α(H)-neohopanes > 17α(H)-hopanes, suggesting new maturity parameters that may be useful into the late oil window.

Microbial production and modification of gases in sedimentary basins: A geochemical case study from a Devonian shale gas play, Michigan basin

An expanded data set for gases produced from the Antrim Shale, a Devonian black shale in the Michigan basin, United States, has allowed for a detailed examination of the related chemical and isotopic compositional changes in the solid-gas-liquid systems that discriminate between microbial and thermogenic gas origin. In the Antrim Shale, economic microbial gas deposits are located near the basin margins where the shale has a relatively low thermal maturity and fresh water infiltrates the permeable fracture network. The most compelling evidence for microbial generation is the correlation between deuterium in methane and coproduced water. Along the basin margins, there is also a systematic enrichment in 13C of ethane and propane with decreasing concentrations that suggests microbial oxidation of these thermogenic gas components. Microbial oxidation accounts not only for the shift in 13C values for ethane, but also, in part, for the geographic trend in gas composition as ethane and higher chain hydrocarbons are preferentially removed. This oxidation is likely an anaerobic process involving a syntrophic relationship between methanogens and sulfate-reducing bacteria.The results of this study are integrated into a predictive model for microbial gas exploration based on key geochemical indicators that are present in both gas and coproduced water. One unequivocal signature of microbial methanogenesis is the extremely positive carbon isotope values for both the dissolved inorganic carbon in the water and the coproduced CO2 gas. In contrast, the 13C value of methane is of limited use in these reservoirs as the values typically fall between the commonly accepted fields for thermogenic and microbial gas. In addition, the confounding isotopic and compositional overprint of microbial oxidation, increasing the values to typically thermogenic values, may obscure the distinction between methanogenic and thermogenic gas.

Diamondoids and oil are not forever

A measure of the amount of oil destroyed by high temperatures deep in the Earth may help to gauge the depths to which oil extraction remains commercially viable, and so be useful in estimating ultimate oil reserves.A measure of the amount of oil destroyed by high temperatures deep in the Earth may help to gauge the depths to which oil extraction remains commercially viable, and so be useful in estimating ultimate oil reserves.

Recent advances in petroleum isotope geochemistry

Abstract Organic matter is isotopically heterogeneous. Lipids tend to be depleted in the heavy carbon and hydrogen isotopes as compared to carbohydrates and lignite. The C and H isotopic composition of kerogens is determined by many factors, such as environmental conditions during formation and deposition and variability of sources of organic matter. Stable isotope concentrations in mature kerogens are not significantly altered during maturation and hence reflect the variability of the organic matter during deposition. Isotope variations in kerogens therefore reflect to some extent the paleogeographic situation and can be used for paleoreconstruction. A basin facies with anoxia and a marginal facies with detrital input can be differentiated isotopically in the Toarcian of Central Europe. Coals are isotopically heterogeneous due to different isotopic composition of macerals. Inertinite and vitrinite tends to be enriched in deuterium as compared to exinite. Also Boghead coals tend to be depleted in dueterium which possibly reflects various lipid contents of respective precursor materials. Petroleums can be genetically classified through the patterns of covariance of their compound classes. Carbonate and shale sourced petroleums may be additionally differentiated through their deuterium content. The general, world-wide applicability of such genetic differentiations needs still to be proved. Carbon and hydrogen isotope variations in methane are genetically controlled. Bacterial methane in freshwater environments is characteristically depleted in deuterium. Even complex origins of natural gases can be unraveled using compositional and isotopic variations. New analytical developments led to the application of carbon isotope analyses of gaseous hydrocarbons desorbed from sediments for geochemical surface exploration. A discussion reveals that processes such as isotope effects during degassing and bacterial oxidation, as well as indigenous formation of hydrocarbons, may completely obscure the results.